Dehumidification is a process of removing moisture from air. There are several known methods of dehumidifying air. However, the two most common methods use refrigeration and desiccants. In the refrigeration based dehumidification method, moisture is made to condense on a cooling coil, thereby removing moisture from an air stream passing over the cooling coil. In the desiccant based dehumidification, the process employed for dehumidification uses absorption or adsorption. An absorption based process uses either liquid or solid desiccants, whereas an adsorption based process uses solid desiccants such as silica gel, activated alumina, molecular sieve, etc.
Desiccant based dehumidifier systems can be either of the twin tower, cyclic type, or of the continuously rotating type. The air to be dried is generally referred to as process air and the air used to regenerate the desiccant is referred as regeneration or reactivation air.
Refrigeration based dehumidification systems are limited in the moisture they can remove. This is because, below a certain dew point humidity, freezing of the coiling coil occurs, thereby requiring a defrost cycle that makes the system more complex. When the air is dried to the required humidity, it is often too cold for the dehumidified space or process. As a result, this air has to be subjected to a reheating process to raise the temperature thereof to the desired level prior to use.
Desiccant dehumidifier systems, on the other hand, dry the air without cooling it and can therefore achieve very low dew points that are necessary for many industrial applications, without the problems of frosting or freezing. Common examples of desiccant dehumidifier use are in the pharmaceutical area for production of drugs, food processing areas, and a wide variety of manufacturing processes which require air at relative humidity or dew points lower than those that can be technically or economically achieved using refrigeration alone.
Most desiccant dehumidifiers are generally composed of a housing that define two or more sets of plenums (commonly called sectors) such that two or more discrete airstreams can be passed through the wheel. The wheel contains a large number of small axially-disposed passages so two or more airstreams can be passed through the wheel without significant cross-mixing. The walls of the passages are impregnated with the desiccant, providing a large contact area between the desiccant and the airstreams passing through it. A first airstream (the process airstream) is passed through the wheel and is dehumidified by the desiccant impregnated in the wheel. A second airstream (the reactivation airstream) is heated and passed through the wheel to drive out the moisture absorbed or adsorbed in the process sector. The wheel is continuously rotated between the process and reactivation sectors so the process air dehumidification is a continuous process. One or more additional airstreams may be passed through the wheel to improve the dehumidification performance and/or reduce the energy requirements of the dehumidifier.
Desiccant dehumidifiers use a considerable amount of heat energy to regenerate or reactivate the desiccant. Accordingly, over the years, significant attention has been given to attempts to minimize the amount of heat energy required. Typically, these efforts have focused on improvements in the configuration of the desiccant bed(s) or wheel, and the control strategies for capacity control of the desiccant dehumidifier system in response to the moisture load in the controlled space, or the process air.
U.S. Patent Publication Number US 2010/0031528 A1 discloses a process for controlling moisture content of a supply gas that is used for drying a product. The process described in this document comprises heating the supply gas if required, determining its temperature and moisture content, and then contacting it with a rotating desiccant wheel, and recovering the dehumidified supply gas. The rotation of the desiccant wheel is controlled using the data relating to the gas temperature and moisture content in combination with the corresponding sorption isotherm of the desiccant. This document stipulates the use of a closed loop of superheated steam as the regenerative medium in order to reduce the high energy consumption of zeolite regeneration. While the document refers to the use of a pressure transmitter to monitor and ensure constant gas flow by the fan and a special transmitter to measure the moisture content of the supply gas, there is no disclosure of the specific means that are used to measure either temperature or moisture content. The method used to determine either temperature or moisture content of the supply gas are therefore, necessarily limited to those that can be used with closed loop steam supply systems.
U.S. Pat. No. 5,188,645 discloses a method and apparatus for dew point adjustment using a dry desiccant dehumidifier. The method disclosed in this document does not use any control mechanism for ensuring determination of moisture or temperature values, and appears to rely instead on providing temperature values that are pre-determined.
U.S. Pat. No. 7,690,582 discloses a humidity control apparatus wherein the amount of heat exchange between the first and second air streams and the amount of moisture exchange between the first and second air streams are varied by changing the rotating speed of the desiccant wheel. Two fixed wheel speeds are used, one for dehumidification of air during summer weather and the other for heating and humidification during winter weather. The humidity control apparatus is switched between two positions—dehumidification operation and humidification/heating operation. The invention of this patent specifically relies on avoiding the use of switching valves. The measurement of temperature of the air that is used appears to be carried out only once, and appears to be a function of pre-determined parameters. This does not provide flexibility in the system operation.
Japanese Patent Publication Number 2010-110736 discloses a method for improving the operating efficiency of a dry desiccant dehumidifier while maintaining a constant dew point humidity of the air at the outlet thereof. The method disclosed in this document involves controlling the reactivation airflow such that the average reactivation outlet temperature is measured and maintained at a fixed value. According to this document, the desiccant is deemed completely reactivated if the average reactivation air outlet temperature is maintained at a fixed value. The document also describes a method and apparatus for controlling the operation of a dry desiccant unit with a purge sector located sequentially between the reactivation and process sectors. The air flow through the purge sector is concurrent with the reactivation airflow and counter current with process airflow. A portion of the process discharge airflow is used for the purge airflow. The airflow through the purge sector is controlled so a constant air temperature is maintained at its discharge. The desiccant rotor speed may be adjusted in proportion with the reactivation airflow. The purge sector uses the residual heat in the wheel for a portion of the reactivation process. Average reactivation and purge discharge temperatures are used. The method and apparatus of this document rely almost exclusively on control of the reactivation air flow to maintain the average reactivation outlet temperature at a predetermined fixed value. The method herein does not provide the necessary flexibility of operation that is desirable and does not enable dynamic control. Japanese Patent Publication Number 2010-247041 describes a method for achieving apparently highly stable variable control operation in controlling a dehumidifier operation. In this method, the average dew point temperature of the supply air may be detected and controlled to satisfy requirements conventionally. The system controls the number of rotations of the rotor or regenerating temperature of a dehumidifier of an adsorbing rotor system according to change of dehumidification load, etc. The method is applicable to a dry desiccant dehumidifier with a purge sector located sequentially between the reactivation and process sectors. This document essentially relates to a method by which the dehumidification load is inferred by measuring the average air temperature rise through the process sector. Controlled variables may include rotor speed, reactivation airflow and temperature, purge sector airflow and process airflow. Purge sector airflow may be in either direction and purge air source may be from process air supply or process air discharge. Again, the method herein does not provide the necessary flexibility of operation that is desirable and does not enable dynamic control due to its almost exclusive reliance on measuring average air temperature rise through the process sector to ensure that this is kept at a predetermined value.
Japanese Patent Publication Number 08-141352 discloses a method to diagnose the degradation of a rotor continuously and forecast time for the replacement of the rotor. The method involves measuring the average outlet temperature of regenerated air in a second stage dehumidifier to diagnose the degradation of the rotor on a first stage dehumidifier. The method and apparatus of this—disclosure is with respect to a dehumidification system having two dehumidifiers in series with the first dehumidifier preconditioning ambient air that is at least a portion of the inlet air for second dehumidifier. The essence of this disclosure consists of inferring the amount of dehumidification occurring in the first dehumidifier by measuring the temperature drop of the reactivation air through the second dehumidifier. The patent is specific to systems with two dehumidifiers in series, and lacks the necessary flexibility that is desirable for solid desiccant based dehumidification systems.
Japanese Patent Publication Number 2001-099451 discloses a method and apparatus wherein the heat amount required for reactivation in a dehumidifier is minimized and wherein a rotor is reactivated with heated air. In this disclosure the rotor is heated with reactivation air, moving in two or more reactivation sectors. The rotor temperature immediately after having moved to the reactivation section is low, but it is heated while moving, and the reactivation air inlet temperature is increased. A temperature distribution is formed in the direction of rotation of a desiccant rotor in a reactivation section. The method and apparatus of this disclosure essentially relates to a dehumidifier having two or more reactivation sectors with progressively higher inlet air temperature in succeeding sectors. Another embodiment shows multiple individual 100% purge sectors paired with corresponding reactivation sectors. Again, the disclosure of this document does not provide any solution to achieving flexibility of operational control in a dynamic manner in solid desiccant based dehumidification systems.
U.S. Pat. No. 6,751,964 discloses an integrated dry desiccant-refrigeration air-conditioning system. The apparatus includes a mechanism for varying the rotational speed of the desiccant rotor to control the amount of moisture removed from the supply airstream or heat transferred to the supply airstream. The scope of the disclosure of this document is apparently to allow the desiccant wheel to be used as a dehumidifier in the summer and an enthalpy recovery wheel in the winter. The objective and purport of this document attempts flexibility but in a different scenario from that required for control operation of a solid desiccant based dehumidifier for minimizing heat energy consumption.
PCT International Publication Number WO 2004/055443 A1 discloses a cooling and dehumidification system including at least one evaporator, at least one variable-speed refrigeration compressor, and at least one condenser and a single desiccant wheel. At least a portion of the air cooled by the evaporator passes through one portion of the desiccant wheel and at least a portion of the condenser air passes through the other portion of the dehumidifier. The speed of the compressors is controlled by at least one condition of the supply airstream, the reactivation airstream and/or the refrigeration system. Variable volume of condenser airflow (hence reactivation airflow) is claimed, for the purpose of maintaining a constant reactivation temperature based on the amount of heat available from the refrigeration system. Variable desiccant rotor speed is not mentioned.
US 2008/0108295 discloses control systems for recovery wheels, ventilation systems with recovery devices, and method of controlling recovery wheels in a ventilation system, methods of controlling a ventilation system to reduce energy consumption, methods of reducing energy consumption of a ventilation system. This document discloses measuring pressure differentials and using the information so obtained to modify the rotor speed. However, the document does not mention modifying the measured parameter itself.
As is evident, while various attempts have been made to provide improvements in heat energy consumption during use, hone of these methods have been successful in providing the desired flexibility of operational control for solid desiccant based dehumidification systems. In fact, it appears that attempts in the art to minimize heat energy consumption during operation of a solid desiccant based dehumidifier have focused on specific situations, rather than attempt an holistic and therefore flexible control system and method.